In terms of gameplay it's a definite improvement from 3. You now have three new characters with their own distinct fighting styles and personalities. Akiyama and Saejima in particular are great additions. It's impressive how much they managed to add only a year after the previous game.

The story on the other hand..
It starts out promising enough, but by the end it's become the most ridiculous in the series by far. That wouldn't be such a problem by itself, but the plot twists get out of control, and one in particular almost ruins Saejima's character (I still love him). It's unnecessarily complicated and ends up being pretty unsatisfying, despite having some very cool moments, such as when all four main characters team up to kick ass. Kiryu ends up being sidelined in his own game, but that was probably to be expected with this many new characters.

The last boss is complete bullshit.